Hi,
Welcome to the community! 
If your project is named sketch, you should have the following folder structure :
sketch
βββ data
β   βββ flower0.jpg
β   βββ flower1.jpg
β   βββ flower2.jpg
βββ sketch.pde
1 directory, 4 files
Your images need to be in a data folder inside your sketch folder, then it should work.
If you are on Linux or Mac, can you use the tree command in the terminal (used above) to show the file structure of your project?